(derived-mode-all-parents): Speed up with a cache

(derived-mode-all-parents): Speed up with a cache

Most uses of the mode hierarchy don't really need to construct the
list, they just need to iterate over it.  With single inheritance
we could do it just by jumping up from a mode to its parent,
but to support the upcoming multiple inheritance we'd need a more
complex and costly iterator.
Luckily, the inheritance graph is mostly static so we can cache
the list of all parents, making `derived-mode-all-parents` cheap
enough to be the basis of iteration and keeping the API very simple.

* lisp/subr.el (derived-mode-all-parents): Cache the result.
(derived-mode--flush): New function.
(derived-mode-set-parent): Use it.